1. Field of Invention
This invention relates to glass-ceramic composite packages for integrated circuits in general, and in particular to metal conductive compositions used for circuit traces and vias to form the interconnective thick film wiring.
2. Description of Prior Art
Multi-layer ceramic substrates for mounting integrated circuit chips generally comprise alternating layers of metallic circuits and ceramic insulating layers to form three dimensional interconnect circuits. The substrates are produced either by a thick film printing method or a green sheet lamination method.
The thick film printing method has been used to fabricate hybrid circuits and multi-layer printed interconnect boards. In this process, metal powders and ceramic powders are formulated into metal and dielectric (insulator) inks and then alternately screen printed onto a fired ceramic base. Generally two or three printings of dielectric material are required for every insulating layer and the circuit must be fired after each printing process. Thus, this method is very time consuming because of the large number of printing and firing steps required. The method is also prone to low production yields and is limited in the density of interconnect circuitry. Ceramic layer hermeticity is a major problem affecting yields and is a direct result of using screen printing methods to form insulating layers. In addition, conventional metal pastes contain active bonding agents to promote adhesion to ceramic substrates (e.g., lead borosilicate glass and bismuth oxide) which function acceptably in air fired applications, but which are problematic in nitrogen firing applications.
According to the green sheet lamination method, green ceramic sheets on which metal circuits have been printed are successively laminated and then co-fired to form a monolithic interconnect structure (package). Generally, the ceramic green tape is fabricated by the doctor blade casting process from a slurry containing a mixture of ceramic powders, thermoplastic resin, solvents, and other additives (dispersants, plasticizer). Polyvinyl butyral (PVB) is the most commonly used resin system for tape formation. The green tape is blanked into sheets and registration holes are punched. Via holes, which in the final package serve as vertical interconnects between layers, are punched using fixed tooling or a numerically controlled punch press. The via holes are filled and circuit trace patterns are printed using the desired metallization compositions. The individual sheets are then stacked in the proper sequence and laminated to form a solid, composite laminate. The laminate is fired to decompose and remove the organic binder and to sinter the ceramic and metal particles, thus forming a dense body containing the desired three-dimensional wiring pattern.
Aluminum oxide, because of its excellent electrical (insulating), thermal, and mechanical (especially strength) properties has been the ceramic of choice for such substrates. These ceramic bodies, generally containing 4-10 weight percent glass, require sintering temperatures above 1500.degree. C., which thus necessitates the use of refractory metals such as molybdenum or tungsten for the wiring. These metals have poor electrical conductivity as compared to highly conductive metals such as copper, and secondly, they require the use of strongly reducing atmospheres during co-firing necessitating expensive furnace systems. Alumina has been an adequate dielectric material for microelectronic packaging in the past; however, the advent of higher frequency and higher speed devices has made clear the deficiencies of the current materials systems. Al.sub.2 O.sub.3 has a relatively high dielectric constant of about 9.8, causing high signal propagation delay and low signal-to-noise ratio (crosstalk). Furthermore, alumina has a thermal expansion of 6.7.times.10.sup.-6 /.degree. C. (20-200.degree.) C. range) as compared to about 3.0-3.5.times.10.sup.-6 /.degree. C. for silicon, which represents significant mismatch in thermal expansion and results in design constraints and reliability concerns (e.g., flip chip technology). Furthermore, the binders used to fabricate green tape do not decompose cleanly during firing at low temperatures (200-600.degree. C.) in reducing atmospheres utilized; significant graphitic carbon is generated which requires a high temperature burnout treatment (1100-1200.degree. C.) prior to raising the temperature to the peak firing condition.
Accordingly, there exists a need for a materials system which allows co-sintering of the ceramic with a conductive metal such as copper, gold, or silver. An IC package fabricated from this system would have significantly improved signal transmission characteristics. To this end, a glass-ceramic material sinterable to a high density at temperatures less than 1000.degree. C. is desirable. To allow co-sintering with copper in a non-oxidizing atmosphere, in particular, the binder material must depolymerize and burnout cleanly, which precludes the use of conventional binders such as PVB. PVB or similar polymers would result in a porous ceramic and carbonaceous residue, thereby deteriorating the mechanical strength and electrical insulation. There also exists a need for a metallurgical system that yields good conductivity, adhesion and solderability when co-fired with the ceramic dielectrics. Furthermore, for optimum yields and performance, the bonding agents and ink vehicle system should be compatible with gold, silver/palladium alloys, and copper.
Extensive prior art exists in the area of metallic inks for thick-film conductors; prior art in copper containing inks is especially relevant to the present case, although significant portions of the following discussion apply to air-fired metallization (Ag, Ag/Pd, Au) as well. Examples of prior art include: Suita, U.S. Pat. No. 4,540,604 (9/85); Mitchell, U.S. Pat. No. 4,172,919 (10/79); Hoffman, U.S. Pat. No. 4,070,518 (1/78); and Grier, 4,072,771 (2/78). Suita contains an excellent review of prior part in copper metallization, and is therefore hereby incorporated herein by reference.
Thick film conductor patterns are typically formed on green tape by screen printing of metallic inks; these inks generally comprise a metal powder, glassy bonding agent powders, other solid additives, and a vehicle system (solvents, dispersant, binder (s), and other organics). The ratio of these components determines the rheology and printing characteristics of the ink. Inks typically contain approximately 70-92 weight percent solids with the balance being the vehicle system. Commercially available inks typically employ metal powders and powdered bonding agents having average particle size between about 0.5-5 micrometers. Typical vehicle components are: ethyl cellulose or methacrylates (as binders) and slow drying solvents such as terpeniol, butyl carbitol acetate and butyl carbitol.
A common bonding agent used in the art is lead borosilicate glass containing approximately 45-65% PbO. Bismuth oxide (Bi.sub.2 O.sub.3) is often added to promote glass wetting of the metal, and hence adhesion. Alkali metal oxides (Na.sub.2 O, K.sub.2 O) are also often added to the glass to reduce the viscosity of the glass during firing. Copper-based inks may also contain additional components such as copper oxide (1-4%) to improve adhesion of copper to the ceramic. In addition, the use of refractory metals to aid in copper oxide reduction to copper during firing, and thus to improve subsequent solder wetting, has been disclosed (e.g., see Suita).
Present inks, which were designed primarily for thick film hybrids and multilayer boards, are suboptimal for use in low temperature co-firing processes, especially when nonoxidizing atmospheres are required. While Bi.sub.2 O.sub.3 has been found useful in air fired applications to assist in bonding, it has been found undesirable when used with nitrogen atmospheres, both in co-firing and in post-firing techniques. Bismuth oxide is reduced in such atmospheres to metallic bismuth which causes embrittlement of copper metallization. Alkali ions are potentially harmful in that they not only increase fluidity of the glass during firing, they also enhance migration of copper (in nonoxidizing atmospheres) and silver (in air) ions into the glass through ion exchange processes. As excessive metal migration may degrade electrical properties of the fired ceramic-metal body (package), the concentration of alkali ions should be minimized. In addition, alkali ions may leach out of the glassy bond phase during exposure to moisture and degrade the environmental stability of the package. Finally, copper oxide has been used to enhance adhesion of the metal trace to ceramics, especially alumina; however, bonding mechanisms involving copper oxide reactions may not be operative in the present case. Accordingly, there is a need for an improved metal ink system, which is co-firable at temperatures approximately less than 1000.degree. C. with glass-ceramic substrates.